Operators of gas turbine engines used in generating electricity at powerplants desire to have the most efficient operations possible in order to maximize their profitability and limit the amount of emissions produced and excess heat lost. In addition to maintenance costs, one of the highest costs associated with operating a gas turbine at a powerplant, is the cost of the fuel burned in the gas turbine, either gas, liquid, or coal. For example, a gas turbine engine that operates on natural gas and is designed to produce approximately 170 MW of electricity when operated at base load, or full power throughout the year, typically consumes about 15.4 billion standard cubic feet of natural gas in a year. Increasing the efficiency of the gas turbine will result in an increase in electrical generation capacity for a given amount of fuel burned. Alternatively, if additional electrical generation is not possible or desired, the required level of electricity can be generated at a lower fuel consumption rate. Under either scenario the powerplant operator achieves a significant cost savings while simultaneously increasing the powerplant efficiency.
Attempts have been made to optimize the efficiency of the engine through compressor and turbine airfoil enhancements, improved combustor cooling, as well as attempts to provide uniform flow to combustor components. An example of a combustion system for the prior art gas turbine engine discussed above is shown in cross section in FIG. 1. The combustion system 10 receives its air for mixing with fuel from a compressor outlet 11, which flows into a large plenum 16 adjacent to a plurality of can annular combustion systems. The air is intended to cool the outer walls of combustion system 10, including impingement cooled transition duct 12, which includes an outer impingement sleeve 13.
However, the geometry of the compressor discharge case 14 does not sufficiently direct the air from compressor 15 towards combustion system 10, and the air unnecessarily loses some of its supply pressure.
An attempt to provide the impingement cooled transition duct 12 with a more uniform flow of air is provided in prior art U.S. Pat. No. 5,737,915. A tri-passage diffuser is positioned at the compressor exit to direct the flow into the compressor discharge case in a more uniform pattern in attempt to recover static pressure of the cooling fluid prior to entering an impingement sleeve surrounding a transition duct. While this device may provide a more uniform flow to an impingement cooled device, it does not maximize the pressure recovery possible prior to entering the combustion system, which is a key element to improved engine efficiency and performance.
A significant way to increase the gas turbine engine performance is to provide the turbine with a higher supply pressure. For a combustion system having a known pressure loss, this can be accomplished by reducing the pressure losses to the air that occurs in the region between the compressor outlet and the combustion chamber.